Modern VLSI circuits often contain sets of logical instances which are arranged in a tree-like structure, with the single output of this set being the root of the tree and the inputs (representing the leaves) being interchangeable (associative-commutative) from a logical (functional) perspective. While the choice of how the rest of the circuit connects to the inputs of this tree does not matter from the functional perspective, these connections can greatly affect the quality of the resulting placement and routing. The decisions tend to be made early on in the design process when no physical information is available.